Petroleum or crude oil is a naturally occurring, flammable liquid found in rock formations in the Earth. It is a complex mixture of hydrocarbons of various molecular weights and other organic compounds. The term “petroleum” was first used in the treatise De Natura Fossilium, published in 1546 by the German mineralogist Georg Bauer.

The most common distillations of petroleum are fuels. These fuels include: ethane and other short-chain alkanes, diesel fuel, fuel oils, gasoline (petrol), jet fuel, kerosene, liquefied petroleum gas (LPG). The other derivatives are alkenes (which is manufactured into plastics or other compounds), lubricants (produces light machine oils, motor oils, and greases, adding viscosity stabilizers as required), wax (used in the packaging of frozen foods, among others), sulfuric acid, bulk tar, petroleum coke (used in speciality carbon products or as solid fuel). 

Composition:- In a strict sense, petroleum includes only crude oil.  But in common usage it includes both crude oil and natural gas. Both crude oil and natural gas are predominantly a mixture of hydrocarbons. Under surface pressure and temperature conditions, the lighter hydrocarbons methane, ethane, propane and butane occur as gases.  The heavier ones from pentane and up are in the form of liquids or solids. However, in the underground oil reservoir the proportion which is gas or liquid varies depending on the subsurface conditions.  An oil well produces predominantly crude oil, with some natural gas dissolved in it. The proportion of hydrocarbons in the petroleum mixture is highly variable between different oil fields and ranges from as much as 97% by weight in the lighter oils to as little as 50% in the heavier oils and bitumens.

The hydrocarbons in crude oil are mostly alkanes, cycloalkanes and various aromatic hydrocarbons while the other organic compounds contain nitrogen, oxygen and sulfur, and trace amounts of metals such as iron, nickel, copper and vanadium. Four different types of hydrocarbon molecules appear in crude oil. They are paraffins, naphthalenes, aromatics and asphaltics. The relative percentage of each varies from oil to oil. 

Crude oil varies greatly in appearance depending on its composition. It is usually black or dark brown, although it may be yellowish or even greenish. In the reservoir it is usually found in association with natural gas, which being lighter forms a gas cap over the petroleum. The saline water which, being heavier than most forms of crude oil, generally sinks beneath it. Crude oil may also be found in semi-solid form mixed with sand and water. 

Petroleum is used mostly for producing fuel oil and gasoline (petrol).   84% by volume of the hydrocarbons present in petroleum is converted into petroleum-based fuels, including gasoline, diesel, jet, heating, and other fuel oils, and liquefied petroleum gas. The lighter grades of crude oil produce the best yields of these products because heavier crude oils have too much carbon and not enough hydrogen. Petroleum is also the raw material for many chemical products, including pharmaceuticals, solvents, fertilizers, pesticides, and plastics. The 16% not used for energy production is converted into these other materials.

Petroleum is found in porous rock formations in the upper strata of some areas of the Earth’s crust. There is also petroleum in oil sands. Consumption is currently around 84 million barrels per day. At current consumption levels, and assuming that oil will be consumed only from reservoirs, known recoverable reserves would be gone around 2039, potentially leading to a global energy crisis.

Formation:- According to generally accepted theory, petroleum is derived from ancient biomass. More specifically, crude oil and natural gas are products of heating of ancient organic materials over geological time. Formation of petroleum occurs from hydrocarbon pyrolysis, in a variety of mostly endothermic reactions at high temperature or pressure. Over geological time the organic matter mixed with mud, and was buried under heavy layers of sediment resulting in high levels of heat and pressure.

Crude oil reservoirs:- 3 conditions must be present for oil reservoirs to form:- (1) a source rock rich in hydrocarbon material buried deep enough for subterranean heat to cook it into oil (2) a porous and permeable reservoir rock for it to accumulate in (3)  a cap rock or other mechanism that prevents it from escaping to the surface. Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.

Today, about 90% of vehicular fuel needs are met by oil. The top three oil producing countries are Saudi Arabia, Russia, and the United States. About 80% of the world’s readily accessible reserves are located in the Middle East.